Video coding methods and video decoding methods include some methods which support adaptive block transform; methods defined by the ISO/IEC 14496-10 (MPEG-4 AVC) standard, and the like can be given as an example. When video is coded, the adaptive block transform allows orthogonal transform of blocks while switching various sizes of blocks on a macroblock basis in the video sequence (see, for example, Non-patent Reference 1 and Non-patent Reference 2). This feature provides less coding distortion and better coded image quality than a method which uses only one transform block size. The appropriate transform block size depends on the size of video, the details of the motions in a picture, and the like, and only one transform block size is sometimes sufficient for the picture. In such a case, a flag for disabling use of plural transform block sizes (i.e., enabling use of a fixed transform block size) is set at the beginning of a sequence or a picture so that the transform block size is switched from one to another only when the flag is off.
There are two types of conventional methods, namely an implicit mode and an explicit mode, for notifying a transform block size to be used for a macroblock. In the implicit mode, a motion compensation block size is used to determine a transform block size. For example, in the case where a motion compensation block size is equal to 8×8 pixels or larger than 8×8, the transform block size is 8×8.In the case where a motion compensation block size is smaller than 8×8, the transform block size is 4×4.
In this mode, an additional syntax element indicating the transform block size is not transmitted at the macroblock level. Therefore, information indicating the transform block size does not need to be coded, and thus the number of bits indicating the transform block size can be reduced. However, since the transform block size is not always linked with the motion compensation block size, this mode does not always allow a selection of an appropriate transform block size, compared with the case where a transform block size varies from one macroblock to another.
The other mode is an explicit mode for transmitting additional information indicating a transform block size for a macroblock at the macroblock level. As an example, a flag can be set at the macroblock level which is larger than or equal to an 8×8 motion compensation block. This flag indicates whether a transform block size is 8×8 or 4×4.If the motion compensation block size is smaller than 8×8, the transform block size is fixed to 4×4.This mode allows an encoder to explicitly set a transform block size for each macroblock in a picture, thus allowing it to always select the best transform block size. However, since a transform block size needs to be coded for each macroblock, there is a problem in that additional bits are needed to code the transform block size.
Non-patent Reference 1: ISO/IEC 14496-10 Advanced Video Coding First Edition: Dec. 1, 2003 (H.264/AVD standard)
Non-patent Reference 2: Draft Text of H.264/AVC Fidelity Range Extensions Amendment: JVT-L047 July, 2004 (a draft of H.264/AVC Amd standard)